Inquiring minds want to know ...
This seems like a pretty lousy place to do it, first because of the
contention hit from holding that high-traffic lock any longer than
necessary, and second because every added chance for error between
ExtendCLOG() and TransactionIdAdvance(ShmemVariableCache->nextXid)
gives us another way to fail in the way recently mentioned by Joe
Conway:
http://archives.postgresql.org/message-id/4DBE4E7D.80501@joeconway.com
Even if it's actually necessary to set up that data structure while
holding XidGenLock, I would *really* like the call to not be exactly
where it is.
regards, tom lane